Here is the full list of all 52 GUNGUNGUN achievements.
Get 20 kills with the Stechkin
Get your first 10x combo
Cause 5 enemies to die by way of explosion
Get hit by a heavy
Earn your first 1000 total bounty
Earn 1 Million in total bounty
Save up 2000 in the bank
Save up 10,000 in the bank
Get your first 30x combo
Kill five heavies with the Martini Henry
Get 200 total kills
Get 1000 total kills
Get 5000 total kills
Get 20 kills with the USP9
Get 200 kills with the USP9
Get 500 kills with the USP9
Get 200 kills with the Stechkin
Get 500 kills with the Stechkin
Get 20 kills with the Kalashnikov
Get 200 kills with the Kalashnikov
Get 500 kills with the Kalashnikov
Get 20 kills with the Shotgun
Get 200 kills with the Shotgun
Get 500 kills with the Shotgun
Get 20 kills with the FAL
Get 200 kills with the FAL
Get 500 kills with the FAL
Get 20 kills with the MP5
Get 200 kills with the MP5
Get 500 kills with the MP5
Get 20 kills with the Rhino
Get 200 kills with the Rhino
Get 500 kills with the Rhino
Get 20 kills with the Martini-Henry
Get 200 kills with the Martini-Henry
Get 500 kills with the Martini-Henry
Collect your first hat
Collect 10 hats
Collect 20 hats
Collect 28 hats
Kill 3 bandits with a single Martini-Henry shot
Heal 5 times in a single round
Stand still for 30 seconds straight
Pick up one of each weapon in a single round
Survive for more than 1 minute in a single round
Survive for more than 3 minutes in a single round
Survive for more than 5 minutes in a single round
Score 50,000 points in a single round
Score 500,000 points in a single round
Score 1,000,000 points in a single round